In general, a liquid developer for electrophotography is prepared by dispersing an inorganic or organic pigment or dye such as carbon black, nigrosine, phthalocyanine blue, etc., a natural or synthetic resin such as an alkyd resin, an acrylic resin, rosine, synthetic rubber, etc., in a liquid having a high electric insulating property and a low dielectric constant, such as a petroleum aliphatic hydrocarbon, etc., and further adding a polarity-controlling agent such as a metal soap, lecithin, linseed oil, a higher fatty acid, a vinyl pyrrolidone-containing polymer, etc., to the resulting dispersion.
In such a developer, the resin is dispersed in the form of insoluble latex grains having a grain size of from several nm to several hundred nm, and it is important for the latex grains that the grains have uniform grain diameter and do not aggregate, precipitate or accumulate during storage thereof. If much requirements are not satisfied, there may be problems in causing poor reproduction of the images formed, stains of non-image portions or malfunctions of the developing machine, such as clogging of a liquid feed pump, etc.
For overcoming the above problems, a dispersion polymerization method in a non-aqueous system for obtaining insoluble latex grains having fine grain diameter and good monodispersibility has been proposed and has further been studied. For example, an improved method using a soluble dispersion-stabilizing resin is disclosed in U.S. Pat. Nos. 3,990,980 and 4,618,557, and JP-A-1-257969, JP-A-2-74956, JP-A-1-282566 and JP-A-2-173667 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and a method for modifying the surface of latex grains by using a compound having a physical and chemical interaction with the monomer to be insolubilized is disclosed in U.S. Pat. No. 4,842,975, JP-A-62-151868, JP-A-1-237668, JP-A-2-168276, JP-A-2-116858 and JP-A-2-186351. These methods are described as being useful for improving degree of grain dispersion, grain diameter, redispersibility and storage stability.
On the other hand, studies for putting practical use or development of direct lithographic printing system using electrophotographic technique has recently been made actively. The system comprises forming a toner image on the surface of an electrophotographic light-sensitive material by electrophotographic system, and treating the resulting light-sensitive material to render the non-image portion thereof hydrophilic thereby producing a lithographic printing master plate.
Typical methods for rendering the non-image portion hydrophilic include a method for dissolving out the non-image area with a treating solution and utilizing hydrophilicity of the surface of the support under the electrophotographic light-sensitive layer, and a method for modifying an oleophilic property of the surface in the non-image portion of the light-sensitive layer to a hydrophilic property.
Recently, as an improved latter method for conventional oil-desensitization treatment of photoconductive zinc oxide in the light-sensitive layer, JP-A-62-21669, JP-A-1-191157 and JP-A-2-15277 disclose that hydrophilicity of the non-image portion can be increased by converting the binder resin used in the light-sensitive layer to a hydrophilic resin. In this system, it is important that the toner image portion remains unchanged upon treatment with the treating solution. Such a property is hereinafter referred to "resistivity" to the treating solution.
When a conventional liquid developer containing insoluble resin grains having good redispersibility is used in the above-described electrophotographic plate-making system and the non-image portion is sufficiently rendered hydrophilic, the toner does not show sufficient resistivity thereby sometimes causing loss of image portions. Such problems occur, in particular, in toner portions having small areas such as fine lines and letters or dot areas, and the quality of prints obtained from such a printing plate is deteriorated.
On the other hand, insoluble latex grains having high resistance to the treating solution, for example, styrene type latexes, possess sufficient resistivity, but these resins have low charge stability and redispersibility as toner grains and also provide complicated problems in apparatus since these resins require a high temperature and long time fixing procedure due to poor fixing property of the toner grains.